Technologies for improving the efficiency of frequency utilization include an Orthogonal Frequency Division Multiplexing (OFDM) technology, a Wavelength Division Multiplexing (Nyquist-WDM) technology, etc. In the OFDM technology and the Nyquist-WDM technology, wavelengths are multiplexed with a narrower wavelength interval for transmission. In a system that is known as a Superchannel, a plurality of sub-channel signals are multiplexed by using the OFDM technology or the Nyquist-WDM technology and are regarded as a single signal in a transmission system of 100 Gbps or higher. In the Superchannel, when the sub-channel signals are multiplexed in light domain, differences in power level are induced among the sub-channel signals within the Superchannel due to variation in loss or the like in an optical filter or an optical coupler of a multiplexer portion. Differences in optical power level among the sub-channel signals within the Superchannel are induced due to wavelength dependency of an optical amplifier in a transmission path, filtering in Add/Drop of a neighboring channel or the like.
A related technology is disclosed in Japanese Laid-open Patent Publication No. 2011-082749.